Department of Endocrinology, University of Milan, Via A. del Sarto 21, 20129 Milan, Italy (Received 8 May 1978) It is well established that the rat prostate gland converts testosterone mainly into 5α-androstan-17β-ol-3-one (5α-dihydrotestosterone, 5α-DHT) and to a lesser extent into 5α-androstan-3α,17β-diol (5α-tetrahydrotestosterone, 5α-THT). This occurs, both in vivo and in vitro, through the action of a 5α-reductase and a 3α-hydroxysteroid dehydrogenase system (Baulieu, Lasnitzki & Robel, 1968; Bruchovsky & Wilson, 1968; Gloyna & Wilson, 1969; Kniewald, Massa & Martini, 1971). It has also been recognized that, although the 5α-reduction of testosterone is an irreversible reaction, the reduction of 5α-DHT to 5α-THT is reversible (Becker, Grabosch, Hoffmann & Voigt, 1973; Cresti & Massa, 1977). Consequently, the question has been raised as to whether the biological actions of 5α-THT are attributable to the compound as such or to 5α-DHT. At the anterior pituitary level, 5α-reductase activity is increased by castration and decreased